For recent years, GPS (Global Positioning System), VICS (Vehicle Information and Communication System), ETC (Electric Toll Collection System) and others have been utilized for smooth running of a vehicle by performing communication using a radio wave between in-vehicle communication equipment and external communication equipment.
As an example of the antenna of such in-vehicle communication equipment used in these systems, an attempt has been made to affix an antenna device on the front windshield of a vehicle, the antenna device including a microstrip antenna (hereinbelow, referred to as MSA). However, transmitted power or received power is deceased since, e.g., reflection of a radio wave is generated by the front windshield because of communication with external communication equipment through the front windshield. Specifically, there has been a problem that a portion of the radio wave radiated from an MSA is reflected on an interface of the front windshield to generate a reflected wave, and that the reflected wave interferes with a radiated wave from the MSA to reduce the gain of the antenna device.
In the prior art, it has been possible to prevent the gain of an MSA from being reduced by using a positioning spacer to limit the position of the MSA and disposing the MSA in the vicinity of a position apart from a front windshield by a distance of an integral multiple of a reference length, the reference length being a length obtained by multiplying the wavelength of a half of the wavelength of a radiated radio wave by a correction constant, as disclosed in JP-A-2002-246817.
JP-A-2002-252520 has disclosed a planar antenna, which has a patch conductor and a grounding conductor disposed only on a single surface of a dielectric substrate. In this planar antenna, the patch conductor is disposed in a certain pattern on the single surface of the dielectric substrate, and the grounding conductor is disposed around the patch conductor, having a certain gap (slot) interposed between both conductors. This planar antenna is called a coplanar patch antenna (hereinbelow, referred to as CPA).
JP-A-5-63423 has disclosed a planar antenna, wherein a conductor layer for a radiating element, a dielectric layer and a grounding conductor layer are disposed on at least one portion of a windowpane for a vehicle in this order from the bottom as “a planar antenna for a vehicle”, and wherein the conductor layer is connected to an input terminal of an amplifier disposed in the vicinity of the antenna. This planar antenna is fabricated by using silver paste for the conductor layer for a radiating element and the grounding conductor layer, using a dielectric material, such as glass, a resin or a ceramic material, for the dielectric layer, printing each of the paste and the dielectric material as a thick film and baking the printed films.
However, the fabricating process is complicated since it is necessary to repeat printing and drying when a multilayer is applied as a thick film to a windowpane by printing. When printing for each of the layers is successfully performed, huge equipment is needed since a printer and a dryer are needed for fabrication of each of the layers. Additionally, it is difficult to simultaneously bake the respective layers in a sufficient manner in a case where the respective layers are printed in a multilayered structure so as to have a shape optimum for a windowpane for a vehicle. Although it is disclosed that a metal plate-like material, a sheet-like material or a film-like material is bonded by an adhesive, antenna characteristics are different from desired characteristics because of the presence of an adhesive layer.
Although it is described that the respective layers can be laminated so as to have a total thickness of hundreds of μm or below, it is difficult to have a resonant structure and to increase radiating efficiency in a microstrip antenna structure when the dielectric layer is too thinner than the wavelength of a radio wave. When an attempt is made to increase the dielectric constant of the dielectric layer and to make the dielectric layer thinner, there has been caused a problem that since an increase in the dielectric constant generally increases dielectric loss, the radiating efficiency as an antenna decreases, and the bandwidth is made narrower, with the result that the antenna device is not suitable for receiving a feeble radio wave from, e.g., an artificial satellite.
JP-A-2002-237714 has disclosed in FIG. 6 a patch antenna device, wherein spacers are disposed on a substrate with a grounding conductor disposed thereon, and wherein a patch conductor made of a metal plate in a square shape is supported by the spacers. In this prior art, the patch conductor is not disposed on a dielectric substrate. This causes a problem in that it is difficult to mount the antenna device when the antenna device is applied to a vehicle or the like.
JP-A-8-265038 has disclosed in FIG. 8 an annular microstrip antenna, wherein an island-like conductor is disposed inside an annular patch conductor disposed on one of the surfaces of a dielectric substrate for performing impedance matching. In this prior art, a grounding conductor is disposed on the other surface of the dielectric substrate, and a center conductor of a coaxial cable is passed through a hole formed in each of the dielectric substrate and the grounding conductor, and a leading edge of the center conductor is connected to the island-like conductor. This causes a problem in that it is difficult to mount the antenna device when the antenna device is applied to a vehicle or the like.
U.S. Pat. No. 6,593,887 has disclosed in FIG. 2 and the like a patch antenna, wherein a patch conductor and a grounding conductor are disposed so as to be apart from each other, and wherein a conductor for electromagnetic coupling is disposed so as to extend toward the patch conductor, passing through a hole formed in the grounding conductor. However, this prior art fails to disclose a specific structure as an antenna device as a whole. This causes a problem in that a mounting means is vague in terms of application of the antenna device on a vehicle or the like.
When an antenna device including the above-mentioned MSA is affixed to a front windshield, the MSA needs to be disposed in the vicinity of a position apart from the front windshield by a distance of an integral multiple of a reference length, the reference length being a length obtained by multiplying the wavelength of a half of the wavelength of a radiated radio wave by a correction constant, as stated earlier.
Accordingly, a dielectric substrate with the MSA disposed thereon and the front windshield need to have a thick gap interposed therebetween. This causes problems in that the thickness of the antenna device with the MSA increases, that a driver, who drives a vehicle with the antenna device mounted thereto, is given bad visibility by the antenna, and additionally that the antenna is not preferable in terms of interior design.
The CPA disclosed in JP-A-2002-252520 can be easily disposed on a front windshield, a backlite or the like since the antenna element comprises a conductor disposed on one of the surfaces of a dielectric substrate. However, it is necessary to use a connector for deriving a received signal from the CPA disposed on the front windshield or the backlite, and to directly solder a coaxial cable, for example. This causes a problem in that the antenna is not necessarily practical in terms of manufacturing efficiency and cost.
When a CPA is mounted to a vehicle for communication with an external communication equipment, the antenna has directivities in two directions of both surfaces of a dielectric substrate. This causes a problem in that a signal cannot be always transmitted or received with good efficiency. From the viewpoint of the problems stated above, it have been demanded to provide an antenna device for a high frequency band, which is made smaller, thinner, more efficient and more inexpensive than the conventional antenna devices.